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Topic: Adding LED (or?) to shield fan help (Read 4199 times)

On an LED, the cathode is typically the shorter lead. General rule of thumb is most LED's run on 2 volts. Series resistors are typically 100 ohms per volt you want to drop. For instance starting with 12 volts, you want to drop 10 volts (to get to the 2 volt operating level), so 10 x 100 would be 1000. A 1000 ohm series resistor will get you close. Adjust that up or down to increase or decrease brightness (within reason or you will toast the LED)

Adjust that up or down to increase or decrease brightness (within reason or you will toast the LED)

Also, look up the specs of the fan you want to use and see what the maximum sink current of the tach transistor is. Do not exceed that. If the max = 10mA (.01A) then R = 10V/.01A = 1K Do not go lower than 1K.

OK, grabbed a fan when I got home and wired it up with the LED in the tach wire. When I stopped the fan, guess what, the LED was on. I then slowly turn the fan by hand and the LED went off. This happened twice during one revolution of the fan. (On-off-on-off). Checking the fan specs, I saw that the tach output is a hall effect transistor. This means that when the fan stops with the magnet over the transistor, it is on. (electronic reed switch). This makes sense as the tach output two pulses per revolution

I checked the fan I am currently using and it's tach out is a bit different.

Looks like W9LRT missing pulse detector might be the way to go. Let me see if I have a 555 around here somewhere.

With what you said about the fan dupreezd, why would a 555 (had to look that up) be needed if the LED does in factturn off (or on) when it comes to a stop? please? I have also looked at the 3 and 4 pin fans online and both thesense (tacho) and PWM have pulses to them, would the PWM work better then the Tach in this instance?

The 555 timer chip can be setup as a "missing pulse detector" to monitor the tach output of the fan. The fan stops and no more pulses out of it. The PWM (Pulse Width Modulation) lead of the fan is an input to it. A throttle so to speak.

Lots of fun. I enjoy projects like that. Just be sure to have all your parts gathered, assembled and tested first before taking your station down to minimize down time.You can Google "555 as a missing pulse detector" and click on "Images" to get lots of ideas and answers.I think you mentioned that you're using a computer power supply and if so you may be able to use a standard power cable assembly to cut and splice your new circuit and plug right in. [ You are not allowed to view attachments ]

Sorry, I could not find a 555 at my house. But looking at the circuit, I am starting to doubt if it will work.

Here is my reasoning:- the fan can stop in any of two positions. If it stops in the 'on' position, the tach output will be low, this will turn on the transistor before the 555, preventing the capacitor to charge. This will put the output (pin 3) of the 555 high.If the fan stop in the 'off', the tach output will be high, turning off the transistor and the capacitor charge up and pin 3 goes low.Which is what we have now.

I still think that my idea above for using a cheap automotive tachometer gauge is a viable solution, and as a side benefit you can read the fan's RPM to see if it is slowing down due to impending failure.....

^^ True. And since the prospect of bring another wire down the mast to connect the gizmos has already been broached, the gauge could be mounted low, or inside an enclosure. Power can be supplied at the bottom with only the sensor leads extended. They come in analog or digital too.

Here is a picture of how to wire it up. Of course in the final one, the component wires will be cut much shorter to make it neat and small. You can also mount it on a piece of veroboard [ You are not allowed to view attachments ]

Note- on the diode there is a grey band, that is the cathode. Make sure it goes the way as in the picture. [ You are not allowed to view attachments ]

Yes, those will do. They are 1%, so 4 bands to read the value. Makes it a bit more difficult to read if you are not used to it. The 'normal' ones, 2,5,10% only have 3 bands, (number, number, multiplier).